1. Field of the Invention
The present invention relates generally to valves for controlling fluid flow and, more particularly, to an element having high strength and both corrosion and erosion resistance for use in a valve. Specifically, the present invention relates to a metallic element having a non-metallic, cylindrical insert secured therein to provide resistance to corrosion and erosion, wherein a coupling mechanism on a metallic portion of the element provides a high-strength element-to-shaft connection. The present invention also encompasses a method of manufacturing a high-strength, erosion and corrosion resistant element for a valve.
2. State of the Art
Many industrial processes consume, or make use of, fluids that may be either highly corrosive, highly abrasive, or both. Corrosive fluids include a broad array of chemicals that may be corrosive to both ferrous and nonferrous metals, as well as other materials. Abrasive fluids include slurries comprising a primary fluid or fluid mixture in which solid particulate matter is suspended. Particles of contaminates carried by an otherwise non-abrasive fluid may also cause erosion. Also, industrial applications often necessitate the delivery of corrosive and/or erosive fluids at high flow rates, high temperature, or both. Industrial processes, as well as scientific or laboratory applications, consuming or making use of corrosive or abrasive fluids--whether at high flow rate or high temperature--require fluid delivery systems adapted to function in severe operating environments.
Industrial fluid delivery systems routinely include one or more fluid valves configured to control the rate of, or completely terminate, fluid flow through the system. These fluid control valves must be constructed of components adapted to withstand the severe operating environments created by corrosive and/or erosive fluids flowing at high temperature or high flow rate. High temperatures may increase the rate at which a fluid chemically attacks (i.e., corrodes) internal components of a valve, and high temperatures may also subject a valve to thermal stresses, especially if thermal cycling is present. Process conditions in the fluid may dictate high pressure drops or high flow rates, subjecting the valve to higher stresses.
A type of valve commonly employed in industrial fluid transportation systems is the ball valve. A conventional ball valve is shown in cross-section in FIG. 1. The conventional ball valve 1 includes a ball or ball element 10 configured to control the rate of fluid flow through the conventional ball valve 1. The ball element 10 comprises a generally spherical body 12 having a cylindrical-shaped fluid passageway 14 extending therethrough and defined by an interior surface 16. Fluid passageway 14 defines a flow path through the ball element 10. The direction of fluid flow through the conventional ball valve 1 and fluid passageway 14 is indicated generally by an arrow 5. The ball element 10 further includes a coupling mechanism 18 configured for attachment of one end of an actuation shaft 20 to the ball element 10.
The conventional ball valve 1 also includes a housing 30 having an inlet 32 and an outlet 34. The inlet 32 and outlet 34 each define a generally cylindrical hole having a diameter of substantially the same size as a diameter of the fluid passageway 14 extending through ball element 10. Supporting the ball element 10 within the housing 30 are seats or seals 40. Each seat 40 comprises a generally cylindrical-shaped structure including a cylindrical aperture 42 extending therethrough and further including a circumferential seating surface 44. The diameter of the aperture 42 of each seat 40 is substantially the same as the diameter of the fluid passageway 14 extending through the ball element 10. The circumferential seating surface 44 of each seat 40 contacts the spherical body 12 along a continuous, circumferential contact region 90. Biasing elements 50 may elastically bias the seating surface 44 of each seat 40 into contact with the ball element 10. The interface between the circumferential seating surface 44 of a seat 40 and the outer surface of ball element 10 at the circumferential contact region 90 functions as a seal, preventing fluid present within inlet 32, fluid passageway 14, outlet 34, and apertures 42 from leaking past, or flowing around, ball element 10 and seats 40. The conventional ball valve 1 may also include a shaft seal 22 guiding the actuation shaft 20 into the housing 30 and preventing fluid leakage therebetween.
Rotation or stroking of the actuation shaft 20 and attached ball element 10 effects a change in flow rate through the conventional ball valve 1. In FIG. 1, the conventional ball valve 1 is depicted in the fully-open position wherein the inlet 32, fluid passageway 14, and outlet 34 are substantially concentrically aligned. Rotation of the ball element 10 away from the fully-open position results in decreased fluid flow through the conventional ball valve 1 as the cross-sectional area of fluid passageway 14 that is open to receive fluid flow from inlet 32 decreases, thereby increasing the resistance to fluid flow through the conventional ball valve 1. In the fully-closed position, the ball element 10 is rotated such that no portion of fluid passageway 14 is open to receive fluid flow from the inlet 32 and the flow of fluid through the conventional ball valve 1 is shut off.
Components of the conventional ball valve 1--in particular, the ball element 10--are constructed of metal and typically perform poorly in the severe environments characteristic of erosive, corrosive or abrasive fluid flow. High temperatures and large flow rates further accelerate degradation of metal surfaces within the conventional ball valve 1. To adapt the conventional ball valve 1 for use with erosive, corrosive and/or abrasive fluids, various non-metallic materials exhibiting high resistance to corrosion and erosion have been incorporated into the conventional ball valve 1. One specific approach commonly used by valve designers is to construct the ball element 10 from a ceramic material. Ceramic materials typically have corrosion and erosion resistance properties superior to those of most metals. The seats 40 may also be fabricated of a ceramic or other non-metallic material.
Constructing a ball valve having a solid ceramic ball element may greatly improve the ability of the ball valve to operate in the severe operating environments characteristic of corrosive or abrasive fluid flow; however, use of a solid ceramic ball element typically results in degradation of the structural integrity of the ball valve. Specifically, ceramic materials are less ductile than are metals and, therefore, are much more susceptible to fracture under tensile loads. The reduced fracture toughness of a solid ceramic ball element--as compared to a solid steel ball element--gives rise to a weak linkage between the ball element and an actuation shaft secured thereto. Also, outer surfaces of a solid ceramic ball element oriented generally perpendicular to the flow stream are more susceptible to fracture and cracking due to impact by solid particulate matter present in the fluid flow.
For ball valves incorporating a solid ceramic ball, a conventional ball-to-shaft coupling comprises one end of a metal actuation shaft secured in a mating hole on the ceramic ball element. When torsional loads are applied to the actuation shaft, such a ball-to-shaft connection exhibits high tensile stresses in the ceramic ball element proximate the outer circumference of the mating hole in the ball element where the actuation shaft is inserted. A large pressure drop across the ball valve places a large load on the ball element, thereby increasing the torque load on the actuation shaft and, accordingly, the tensile loads in the ceramic ball element proximate the ball-to-shaft coupling. Build-up of scaling and other contaminates on the surface of the ball element also increase the torque load on the actuation shaft and the tensile loads in the ceramic ball element. Further, if a foreign object becomes wedged between the solid ceramic ball element and the housing or seats during stroking, failure at the ball-to-shaft connection due to tensile stresses on the ceramic ball is nearly certain.
A number of approaches have been suggested to improve the strength of the ball-to-shaft connection between a solid ceramic ball and a metal actuation shaft. U.S. Pat. No. 5,566,923 to Ennis et al., U.S. Pat. No. 5,386,967 to Ennis et al., and U.S. Pat. No. 4,936,546 to Berchem, each discloses a method of increasing the strength of the ball-to-shaft connection between a solid ceramic ball and a metal actuation shaft using mechanical advantage. Generally, a mechanical advantage is obtained using a ball-to-shaft coupling comprised of a plurality of mating pins and holes positioned away from the rotational axis of the ball element. For example, the solid ceramic ball may include a plurality of holes arranged in a pattern on an interface surface, with the metal actuation shaft having a plurality of mating pins extending from one end thereof arranged in a corresponding pattern. Radially spacing the holes on the ceramic ball element away from its rotational axis and using multiple pins and holes allows a larger moment to be transferred to the ball with lower forces, therefore, lower tensile stresses are induced in the solid ceramic ball in the vicinity of each hole. Although reduced, tensile stresses on the ceramic ball element remain. A further drawback of these methods is the difficulty in manufacturing such a ball-to-shaft coupling due to precise alignment and design tolerances that must be maintained between the mating pins and holes.
U.S. Pat. No. 3,949,965 to Samples et al. suggests manufacturing a ball element and attached actuation shaft as a single, integral ceramic structure. Use of an integral ball element and actuation shaft eliminates the ball-to-shaft connection and the stress loads inherent to such a connection. However, an integral ball element and actuation shaft comprised of ceramic as described by Samples et al. is structurally weak as a ceramic actuation shaft cannot withstand high torque loads.
Another conventional method for strengthening the ball-to-shaft connection in a ball valve having a solid ceramic ball element is to employ an actuation shaft having a shaped end or key and a correspondingly shaped slot or keyway on the solid ceramic ball element. For example, the shape of the key and keyway may be square or hexagonal. Use of a mating key and keyway to link a ceramic ball element and a metal actuation shaft does reduce the tensile stresses in the ceramic ball element proximate the keyway; however, tensile stresses are still present in the ceramic ball element which will cause failure.
In view of these shortcomings, there is a need in the art for a ball element with a ball valve exhibiting high corrosion resistance and high erosion resistance that does not exhibit a structural weak link at the ball-to-shaft connection between the ball element and an actuation shaft. Further, there is a need in the art for such a ball element for a ball valve that can withstand high flow rates, large pressure drops, and high temperatures.